The present invention relates to a novel fuel injection valve for a direct injection engine, and in particular, it relates to a fuel injection valve and a direct injection type engine using the same for an automobile.
A gasoline direct injection engine is comprised of a cylinder block, a piston having a piston ring, which is inserted in the cylinder block, and a cylinder head which is in the upper portion of the cylinder block. A combustion chamber is formed in a space surrounded by an internal wall of the cylinder block, an upper surface of the piston and a bottom surface of the cylinder head. Substantially in the center portion of the cylinder head, there is mounted an ignition plug. An air-intake valve and an exhaust valve are provided near the ignition plug. In addition, a fuel injection valve 1 is provided for directly injecting fuel into the combustion chamber. Atomized fuel injected from the fuel injection valve 1 impinges on a concave surface at the top of the piston to be reflected and guided toward the vicinity of the ignition plug which is substantially in the center portion of the cylinder head, whereby realizing a stratified lean burn combustion is realized.
The fuel injection valve of the gasoline direct injection engine, which is installed within the engine cylinder, is exposed to a high temperature combustion gas. In this condition, deposits produced by combustion of gasoline tend easily to accumulate on the tip of the fuel injection valve, thereby distorting a fuel atomization pattern specified to take place within the engine cylinder, consequently resulting in a decrease of its fuel flow rate, and deterioration of a fuel-air mixture, thereby causing its combustion to become very unstable. A cause of the deposits is considered to be due to accumulation of soot produced in the combustion chamber, and a gumlike substance produced by thermal decomposition of gasoline. In particular, when the temperature in the circumference of the fuel injection valve is higher than 160xc2x0 C., the deposits are reported to be easily accumulated. Several methods have been tried for cleaning such deposits by mixing additives into gasoline or by reducing the coarseness of the surface of the fuel injection valve (Jidosha-gijyutsu-kai; symposium preprint 976 (1997-10)). Further, many attempts have been made to drop the temperature of the tip end of the injection valve as disclosed in JPA Laid-Open No. 9-264232. However, it has been difficult by any of these attempts effectively to reduce the deposits. JPA Laid-Open No.9-264232 discloses that the surface of the fuel injection valve is made oleophobic so as to be able easily to remove the deposits, and prevent the decrease of fuel flow. According to this method, a fluoroalkyl compound is reaction-bonded on the surface of the fuel injection valve so as to make its surface oleophobic. Still further, according to JPA Laid-Open No. 7-246365, it is disclosed that the surface of the fuel injection valve is treated by a sol-gel method using a mixture solution of a metal alkoxide and a fluoroalkyl group substituted metal alkoxide which is prepared by substituting a part of alkoxyl group with a fluoroalkyl group, thereby making the surface of the fuel injection valve oleophobic such that the deposits can be easily removed and the decrease in the fuel flow can be prevented. This method includes such one whereby a mixture solution of a fluoroalkyl group substituted metal alkoxid compound and a metal alkoxide is reaction-bonded on the surface of the fuel injection valve so as to make its surface oleophobic, and such ones to form various films as disclosed in JUP Nos.55-116875 and 56-25067. However, these methods are accompanied with a problem to be solved that when the temperature at the tip end of the fuel injection valve exceeds a point at which 90% of the fuel used evaporates, accumulation of deposits progresses on the fuel injection valve so as to reduce the area of opening of the fuel injection valve""s port, thereby decreasing the flow rate of the fuel.
The cause of the production of the deposits is considered, as described in JPA Laid-Open No.9-264232, to be that high residual components in the fuel tend to remain on the surface of the fuel injection valve, and its residual as a core causes subsequent dehydrogenation and polymerization reactions. The prior art method of reaction-bonding the fluoroalkyl compound on the surface of the fuel injection valve so as to be able easily to peel off the deposits is involved with the problem that when the temperature at the tip end of the fuel injection valve is raised as high as to increase the production of the deposits, its effect is reduced.
Further, the method disclosed in JPA Laid-Open No.7-246365 whereby the mixture of the metal alkoxide and the fluoroalkyl group substituted metal alkoxide was baked on the surface of the fuel injection valve so as to render the surface of the injection valve oleophobic thereby improving its deposit peel-off capability, is associated with a problem that when the temperature at the tip end of the injection valve is raised and the production of the deposits increases accordingly, its overall effect is reduced. This cause is considered, as discussed in JPA Laid-Open No.10-159687, to be that the provision of the oleophobic property was insufficient to realize its designed function. Still further, it is necessary for this oleophobic property to exist stably in conditions of a high fuel pressure, high combustion pressure, and high surface temperatures of 150 to 200xc2x0 C. on the surface of the injection valve.
In order to solve the problems associated with the prior art, it is contemplated effective to coat the surface of the fuel injection valve with a fluorine film having a low surface energy, or to reaction-bond a thick film thereon using a fluorine compound having a long chain according to the invention. By provision of such coating or film, the deposits thereon can be cleaned out easily by the fuel of gasoline thereby advantageously preventing adhesion of the deposits thereon. If this object of the invention is realized, a stable combustion pattern designated for a highly reliable gasoline direct injection engine can be achieved. In order to accomplish the object of the invention, there are the following problems to be solved.
A material suitable for this object must be able to exist stably on the surface of the fuel injection valve under conditions of 5-12 MPa of fuel pressures, and 150-200xc2x0 C. of temperatures on the surface of the fuel injection valve, and in addition, must be able to provide a low surface energy with the oleophobic property. Here, the stability (to exist stably) refers to that the material must be nonflammable even if in an environment exposed to the combustion of gasoline for a long time, therefore requiring a high oxidation stability, thermal stability, and gasoline stability, as well as a high adhesion to the surface of the fuel injection valve. Thereby, these problems must have been solved.
The object of the invention is to provide for a fuel injection valve for use in a gasoline direct injection engine, a gasoline direct injection engine and an automobile using the same, which can prevent the deposit produced in the combustion of gasoline to settle on the surface of the injection valve thereof, or which can easily remove the deposits attached thereon.
According to the feature of the invention, a fuel injection valve suitable for use in a gasoline direct injection engine is provided, which can prevent adhesion of the deposit produced in the combustion of gasoline on the surface of the injection valve, and/or easily remove the deposit adhered thereto.
A material of a deposit-resistant film on the surface of the fuel injection valve suitable for use in a gasoline direct injection engine must be such one which can stably exist on the surface of the injection valve which is exposed to an environment of 5-12 MPa of fuel pressure, 150-200xc2x0 C. of temperatures on the surface of the valve under combustion of gasoline, and in addition, which can provide a low surface energy as well as a strong adhesion to the injection valve under such environment.
A surface modifying reagent for forming the deposit-resistant film in order for the same to be used in the aforementioned environment, must be essentially nonflammable thereby limiting its materials to be used. An organic compound which can withstand the above-mentioned environment is preferably a perfluoro compound. This compound is most preferable as a material which can provide for a low surface energy, and is also preferable in the terms of oxidation stability, thermal stability and gasoline resistant stability as well. However, because of its low surface energy, the perfluoro compound has a weak adhesion with a substrate. Hence, it becomes necessary to provide for a compound which has a group to combine with the terminal of the perfluoro compound which bonds with the substrate by reaction. Further, the length of molecular chain in the fluoroalkyl compound used in the prior art is as small as 1 nm or less, therefore, when the deposit is pressed against the surface of the injection valve under the fuel pressure of 5-12 MPa, the deposit is easily caused to pierce through 1 nm thick film of perfluoroalkyl compound to get directly in contact with the surface body to bond therewith. In order to solve this problem, it is contemplated according to the invention that if a thick film of a fluorine compound having a long chain is provided, the adhesion of the deposit can be prevented. However, because the number of carbon in the perfluoro alkyl compounds is generally from 14 to 16 in maximum, it is difficult to synthesize its compound having an increased polymerization.
Hence, we noted to use a polymer of a perfluoropolyether compound as a candidate material which can be stably used in the above-mentioned environment. This perfluoropolyether compound is an average number of molecule weights from 2000 to 8000, and a shape of the compound is looklike yarn ball of more than 1.5 nm in average size (2xc3x97radius of molecule rotation). Then, if a dense film of coating of a perfluoropolyether compound can be formed, the surface of the fuel injection valve can be coated 1.5 nm thick or more in average. Because the surface of the yarn ball of the above-mentioned perfluoropolyether compound is covered by fluorine atoms, it has a low surface energy, thereby preventing adhesion of the deposits, or facilitating peel-off of the deposits. Further, when subjected to an external mechanical pressure, the above-mentioned yarn ball is considered to function as a buffer film. According to this effect, even if the deposit is pressed against the surface of the fuel injection valve at pressures of 5-12 MPa of the fuel, the deposit is considered not to penetrate through the coating of perfluoropolyether compound, thereby preventing its adhesion on the surface of the injection valve. In order for this perfluoropolyether compound to be strongly bonded on the substrate, a most general method will be to provide for alkoxy silane bonded to its terminal.